1. Field of the Invention
The invention relates to a sputter target and backing plate assembly capable of withstanding high sputtering power levels.
2. Description of the Prior Art
Sputtering as a means to deposit thin films of a desired material on a substrate has become important in the manufacture of semiconductor devices such as integrated circuits. In a sputtering system, material to be deposited on a substrate is removed from a sputter target by bombardment of the target with ions. However, ion bombardment not only causes atoms or molecules of the target material to be ejected from the target, it also imparts thermal energy to the target.
Normally, a sputtering system comprises a sputter source, a vacuum chamber, and means for positioning and holding the substrate in proximity to the sputter source. The sputter source normally comprises a target from which material is to be sputtered, means for holding the target in place, means for creating a plasma in the vicinity of the target, means for causing ions in the plasma to bombard the target, and means for cooling the target to prevent overheating.
Various means have been used in the past for holding sputter targets in place within the sputter sources. Such holding means must insure that the target maintains good thermal contact with the cooling means. In a vacuum environment this generally requires good physical contact. On the other hand, sputter targets must be replaced from time to time as the target material is removed and the target has eroded away.
In some sputter sources, annular targets are positioned within a fixed backing plate and retained in place by various hold down features, such as retainers positioned within central openings through the target and fastened to the backing plate. In these sputter source designs, the sputtering targets are independently removable from the fixed backing plates, for replacement of the targets.
In order to cool the targets, cooling water is typically circulated through the backing plate positioned adjacent to the target. In some cases, the thermal contact between an outer rim of the target and the peripheral cooling wall is critical, and is maintained by thermal expansion of the target against the cooling wall. Examples of such sources are shown in U.S. Pat. No. 4,100,055; 4,385,979; 4,457,825 and 4,657,654.
In other versions of sputtering targets, the sputtering target is physically soldered to a backing plate, where the backing plate and the target are insertable and removable, as a unit, into and out of the sputtering system. When the target is replaced, both the spent target and the corresponding backing plate are discarded. The backing plate in such designs typically includes an upper annularly shaped planar surface adapted for flush receipt against the lower surface of the target. The backing plate further includes an annular ring extending downwardly from the planar surface having an outer mounting flange at the lower end of the annular ring.
In these sputtering designs, the targets are physically mounted to the upper surface of the planar backing plate surface, usually by a tin-silver solder film between the lower surface of the target and the planar surface of the backing plate. Cooling of the target is accomplished by circulating water into the chamber formed within the annular ring of the mounting flange, and against the lower surface of the annularly shaped planar surface of the backing plate. This particular design of sputtering target assembly has proven to have several drawbacks.
The annular planar surface portion of the backing plate usually consists of stainless steel, while the sputtering sources consist of aluminum, and thus upon heating, the backing plates and sputter sources have different thermal expansion rates. These dissimilar expansion rates cause thermal stresses at the solder joint and some may fail due to the shear stress caused by the thermal distortion.
It is also common now, to operate these sputtering targets at higher sputtering power levels, in the range of 20-25 KW of sputtering power, in order to achieve high sputtering deposition rates. At these high power levels, the targets are overheated, and melt the solder joint between the targets and the backing plates, causing failures in the sputtering target equipment, resulting in needless downtime of the equipment.
These assemblies are overheating due to the high sputtering power levels at which the sputtering targets are operating, and due to the inadequate cooling capacity of the assemblies. The stainless steel backing plate between the cooling water and the target assembly, increases the resistance of heat transfer, between the heated target and the cooling water, preventing an adequate cooling of the target, required for such high sputtering power levels.
Accordingly, it is an object of the invention to design a sputtering target and backing plate combination which can be operated at higher sputtering power levels.
It is a further object of the invention to design a fixed interconnection between a sputter source and backing plate, having improved heat transfer capabilities.
It is a further object of the invention to design a sputter source and backing plate combination having lower thermal stresses.
Other objects and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims.